NEUTRON FLUX SPECTRUMDOE-HDBK-1019/1-93Reactor Theory (Neutron Characteristics)NP-02Page 34Rev. 0Figure 3 Comparison of Neutron Flux Spectra for Thermal and Fast Breeder ReactorThermalandFastBreederReactorNeutronSpectraThe spectrum of neutron energies produced by fission varies significantly from the energyspectrum, or flux, existing in a reactor at a given time. Figure 3 illustrates the difference inneutron flux spectra between a thermal reactor and a fast breeder reactor. The energydistribution of neutrons from fission is essentially the same for both reactors, so the differencesin the curve shapes may be attributed to the neutron moderation or slowing down effects.No attempt is made to thermalize or slow down neutrons in the fast breeder reactor (liquid metalcooled); therefore, an insignificant number of neutrons exist in the thermal range. For thethermal reactor (water moderated), the spectrum of neutrons in the fast region (> 0.1 MeV) hasa shape similar to that for the spectrum of neutrons emitted by the fission process.In the thermal reactor, the flux in the intermediate energy region (1 eV to 0.1 MeV) hasapproximately a 1/E dependence. That is, if the energy (E) is halved, the flux doubles. This1/E dependence is caused by the slowing down process, where elastic collisions remove aconstant fraction of the neutron energy per collision (on the average), independent of energy;thus, the neutron loses larger amounts of energy per collision at higher energies than at lowerenergies. The fact that the neutrons lose a constant fraction of energy per collision causes theneutrons to tend to "pile up" at lower energies, that is, a greater number of neutrons exist at thelower energies as a result of this behavior.